In many cultures, sexuality was the physiological or emotional drive responsible for physical attachments stemming either from a biological or societal need to bond with a mate. This adaptation primarily facilitated sexual reproduction, however, even those attracted that could not or did not reproduce still had a deep-seated need for the kind of satisfaction a mate provided. In many cases, the ties contributed to causing permanent or long-lasting sexual relationships, which could be monogamous or polygamous. Some species, like Vulcans, had a telepathic bond which formed between mates. (TOS: "Amok Time")

Deltans were also known to project a strong sexual presence even without physical contact, which might have included pheromones and some subconscious telepathic elements. The effects were sufficiently strong as to influence other species. (Star Trek: The Motion Picture)

This drive could also lead individuals to enter a relationship or mate for reasons of satisfaction or emotional fulfillment rather than procreation. Individuals of many species were also known to enter into homosexual relationships. Many individuals encountered in the mirror universe were known to seek out homosexual attachments. (DS9: "The Emperor's New Cloak")

Though homosexual relationships were present in both episodes, homosexuality was never directly discussed.

The Doctor had to upload his program to Seven of Nine, effectively taking over her body and being in full control of it. Seven of Nine was mentally aware of The Doctor becoming sexually aroused in her body while being massaged by Doctor Jaryn. (VOY: "Body and Soul")

In the 24th century, gender-reassignment surgery, otherwise known as a sex change, was known to be performed. In 2374, Dr. Bashir performed such surgery on Quark with no complications or special preparations mentioned. The surgery performed on Quark was more than mere facial cosmetics: Bashir injected female hormones into Quark, and, eventually, Quark removed his clothing to prove that he was a "real" female. The onlooking businessman noted that Quark looked like a female, indicating that Bashir had made physical changes to the chest and/or genital areas of Quark's body. (DS9: "Profit and Lace")

George Takei recalled that during the production of Star Trek: The Original Series he had asked creator Gene Roddenberry why there were no gay or lesbian characters in the series. According to Takei, "He said 'I'm treading a fine tight wire here. I'm dealing with issues of the time. I'm dealing with the civil rights movement, the Vietnam War, the Cold War, and I need to be able to make that statement by staying on the air.' He said, 'If I dealt with that issue I wouldn't be able to deal with any issue because I would be canceled.'" [1]

Ronald D. Moore during the production of Season 6 of Star Trek: Deep Space Nine stated that the show was not interested in stories about homosexuality, or revealing any main character as bi or gay, because it would not really be an issue to them, and so "exploring" it did not hold much promise. He did not disagree that maybe the show should do stories in this vein, but felt that he was more passionate about other issues and therefore should write about that. (AOL chat,1997)